In response to the recent enhanced exhaust emission control for automobiles in North America, Europe, Japan, and other countries, the improvement of engine exhaust emission characteristics is being strongly demanded. As the performance and control precision of catalysts are being improved, exhaust emission from engines is dominant when they are started. Highly precious air-fuel ratio control is also effective in improving the engine exhaust emission characteristics at the time of start. However, exhaust air-fuel ratio feedback control, in which an exhaust air-fuel ratio is detected and the amount of fuel to be injected is then corrected, is usually started at a fixed time (10 to 20 seconds) after the start.
An air-fuel ratio sensor is activated at an early stage and a preheat system is introduced, so there is the prospect that exhaust air-fuel ratio feedback control initiated at the start will be put into practical use. When the fuel vaporization rate is low because, for example, temperature is low or heavy fuel is used, fuel that entered the combustion chamber is exhausted into the exhaust path without being burned. The exhausted fuel is oxidized in the exhaust path and by an exhaust air-fuel ratio sensor probe. When this happens, the exhaust air-fuel ratio is higher than the actual combustion air-fuel ratio. If the exhaust air-fuel ratio is fed back and the amount of fuel is determined according to the feedback, the combustion air-fuel ratio is not properly controlled (the combustion does not approach stoichiometric combustion) and an excessively lean burn thereby occurs, lowering exhaust purifying performance and running operation. To avoid this situation, a technology for initiating the detection of the combustion air-fuel ratio with high precision at the time of start and controlling the combustion air-fuel ratio is necessary.
Accordingly, various control methods and apparatuses have been proposed as in Japanese Patent Laid-open No. 2003-83133 (Patent Document 1) and Japanese Patent Laid-open No. 10-30537 (Patent Document 2).